1. Field of the Invention
The present invention relates to the preparation of organopolysilazanes and/or organopoly(disilyl)silazanes having controlled physicochemical properties, and, more especially, to the preparation of polysilazanes particularly suited for the production of ceramic materials and shaped articles based on silicon nitride and/or silicon carbide, for example fibrous such materials/shaped articles.
2. Description of the Prior Art
Organopolysilazanes and organopoly(disilyl)silazanes, hereinafter designated "polysilazanes", are well known products existing as monomers, oligomers and cyclic or linear polymers, and also as polymer resins. These polysilazanes may be prepared by a wide variety of processes from a broad range of starting materials.
The polysilazanes may in particular be shaped and pyrolyzed into Si.sub.3 N.sub.4, SiC or mixtures thereof. They may also be extruded as continuous filaments, the pyrolysis of which yields ceramic fibers.
They may also be in the form of a more or less thin film, or solid molded pieces, and used as a binder of ceramic or carbon fibers and as a sintering binder for porous ceramic shaped articles. The variety of possible forms of the polysilazanes is one of their principal advantages.
To produce good ceramic precursors, the polysilazanes must be converted, after pyrolysis, into ceramic materials in a high yield by weight. It is therefore necessary that the polysilazane have good heat resistance in pyrolysis, which may be provided in particular by a high molecular weight and/or a high viscosity of up to the solid state at ambient temperature.
The monomers or oligomers produced by the conventional ammonolysis of one or more organochlorosilanes are not good precursors, in particular because of their low boiling point temperatures. Pyrolysis thus entails their distillation, at least if it is not carried out under a high pressure, and consequently the ceramic yields are especially low. It will thus be apparent that serious need exists in this art for macromolecular polysilazane backbones having molecular weights sufficiently high to obviate the above problem.
To this end, the catalytic polymerization of silazanes to produce materials having appreciably improved ceramic yields after pyrolysis has already been proposed to this art.
Such polymerization also makes it possible to produce polysilazanes that are solid at ambient temperature and which melt at higher temperatures, thus being potentially extrudable.
However, these polymerization reactions, although they effectively increase the molecular weight of the polysilazanes, have several disadvantages, principally related to the fact that to date it has not been possible to control and deactivate them when the polysilazane, over the course of the polymerization, has attained adequate physicochemical properties for its subsequent conversion.
Therefore, by controlling the degree of polymerization of the polysilazane, it would be possible to control its melting temperature and consequently its forming temperature, which necessarily must be lower than the temperature of onset of degradation of the polymer.
On the other hand, nonnegligible residual amounts of the catalyst may still be present in the formed and recovered polysilazane, such that even following the polymerization stage proper, the polysilazane can continue to increase in molecular weight at ambient temperature and/or during a subsequent treatment and/or forming. This is manifested principally by an uncontrollable increase in the melting point of the polymer, which may be very deleterious in industrial production (handling of polymer stocks, control of materials for the conversion thereof).